Apparatus for inductively coupling an induction heating coil with a ferrous cooking utensil to thereby electromagnetically heat the contents of the utensil have been widely known and used for many years. In such apparatus, the indication coil is located below a magnetic flux-permeable cooking surface and an alternating current through the coil causes a continuously changing magnetic field to be generated. The magnetic flux of the magnetic field extends through the cooking surface to link with the cooking utensil to cause eddy currents in the utensil and allow it to heat up.
Prior art arrangements for induction heating and cooking appliances include sensing arrangements for determining whether a cooking utensil is in place on the cooking surface above the induction coil before the coil is energized. These sensing arrangements are designed to insure that the high intensity electromagnetic fields which emanate from the induction heating coil are generated only when a utensil is in position overlying the induction coil, thereby limiting the undesirable transmission or leakage of electromagnetic flux into the free space surrounding the cooking appliance.
Various types of sensor arrangements have been used for this purpose. For example, U.S. Pat. No. 3,796,850--Mooreland II et al discloses an arrangement which utilizes a reed switch coupled to two magnets. If a utensil is not present over the induction heating unit, the contacts of the reed switch are forced to close due to the magnetic flux produced by magnets located adjacent the unit. However, if a utensil is placed over the induction heating unit, the magnetic flux is not sufficiently strong to close the contacts of the reed switch and the induction unit is allowed to be powered.
Similarly, the detection arrangement of U.S. Pat. No. 3,993,885--Kominami et al includes a movable magnet, a fixed magnet and a reed switch situated between the two magnets. If a ferrous utensil is placed upon the induction heating unit, the movable magnet is attracted towards the pan and the flux lines near the reed switch are changed allowing power to be supplied to the heating coil.
U.S. Pat. No. 4,013,859--Peters, Jr., utilizes a very low power oscillator coupled to a load sensing coil to indicate the presence of a pan over the heating coil. Furthermore, U.S. Pat. Nos. 3,823,297--Cunningham; 4,016,392--Kobayashi et al; and 4,010,342--Austin include current or voltage detectors which also indicate the presence of a pan above the induction heating coil.
While the above noted patents disclose sensing arrangements which disable the inverter circuit of the induction heating coil in the absence of a utensil on the cooking surface, none of these patents is directed to the problem of disabling the induction heating coil if a utensil is present but nonetheless not centered with respect to the induction heating coil.
This latter situation creates an undesirable condition which results in the leakage of excessive magnetic flux into the space surrounding the cooking surface, which leakage may cause interference with television and radio signals and other communication systems. For this reason, among others, governmental regulating agencies have set limits on the magnetic field leakage of this type attendant to the use of induction heating appliances. Since the intensity of flux leaking into surrounding space increases as a result of operation of an induction heating unit with an improperly centered cooking utensil, it is desirable to provide an arrangement for insuring that operation of the unit takes place only when such utensils are properly positioned over the induction coil.
The aforementioned co-pending applications are directed to arrangements for detecting non-centered placement of a cooking utensil over an induction heating coil. Application Ser. No. 108,087 discloses a sensor arrangement for an induction heating apparatus which monitors the position of a ferrous cooking utensil on a cooking surface and disables an inverter circuit powering the induction heating coil if the utensil is located at an off-center position with respect to the coil, or if no utensil is present upon the cooking surface. The sensor arrangement disclosed in this application comprises a plurality of sets of sensors at successively larger distances from the center of the cooking surface. Each set comprises a plurality of sensors arranged on an imaginary circle substantially equidistant from each other. Each sensor operates to provide an indication of the presence or absence of a utensil directly above the sensor. The sensors are monitored by a logic arrangement which indicates whether a utensil is properly centered based on the fullness or degree of activation of the various sets. If the utensil is not properly positioned, the inverter is disabled and a signal advises the user of this condition. Additionally, the logic circuit determines the size of a properly positioned utensil and generates a signal which may be utilized to alter the output of the heating coil in accordance therewith.
Application Ser. No. 108,086 discloses a utensil detector for an induction heating apparatus which disables the inverter circuit if the utensil is located at an off-center position with respect to the heating coil, if no utensil is placed upon the cooking surface or if an incorrectly sized utensil is placed correctly on the cooking surface. The detector includes three sensor elements spaced 120.degree. apart and situated beyond the periphery of the induction heating coil. The sensor elements monitor the intensity of the magnetic flux in areas adjacent the cooking zone directly affected by the position or size of the cooking utensil. If the combined outputs of the sensor elements do not meet a predetermined criteria, the inverter circuit connected to the induction heating coil is disabled, and the user is alerted to this condition.